ultraviolet radiation from F and K stars and implications for planetary habitability

Now that extrasolar planets have been found, it is timely to ask whether some of them might be suitable for life. Climatic constraints on planetary habitability indicate that a reasonably wide habitable zone exists around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiting such stars would be habitable when biologically-damaging energetic radiation is also considered. The large amounts of UV radiation emitted by early-type stars have been suggested to pose a problem for evolving life in their vicinity. But one might also argue that the real problem lies with late-type stars, which emit proportionally less radiation at the short wavelengths ( < 200 nm) required to split O2 and initiate ozone formation. We show here that neither of these concerns is necessarily fatal to the evolution of advanced life: Earth-like planets orbiting F and K stars may well receive less harmful UV radiation at their surfaces than does the Earth itself.     

Thermodynamics of thermal unfolding of bovine apo-alpha-lactalbumin

Thermal unfolding of bovine alpha-lactalbumin in 10 mM borate buffer at pH 8.0 in the presence of 0.01-1.0 M NaCl was studied in terms of CD ellipticity. The apoprotein changes the conformation from a native-like (N) to an unfolded (U) form, which has an appreciable amount of the secondary structure but no tertiary structure, in the two-state type. Various thermodynamic parameters of the transition were analyzed. The differences in enthalpy and heat capacity between the N and U states are similar to the corresponding differences of the holoprotein obtained with the calorimetric method by Pfeil. It is shown that one Na+ binds with a binding constant larger than 10(2)-10(3) M-1 to a specific site (probably to the Ca2+-binding site) in the molecule and the bound Na+ stabilizes the N form of the apoprotein.     

Thermodynamics and kinetics of the thermal unfolding of plastocyanin

The thermal denaturation of plastocyanin in aqueous solution was investigated by means of DSC, ESR and absorbance techniques, with the aim of determining the thermodynamic stability of the protein and of characterizing the thermally induced conformational changes of its active site. The DSC and absorbance experiments indicated an irreversible and kinetically controlled denaturation path. The extrapolation of the heat capacity and optical data at infinite scan rate made it possible to calculate the kinetic and thermodynamic parameters associated with the denaturation steps. The denaturation pathway proposed, and the parameters found from the calorimetric data, were checked by computer simulation using an equation containing the information necessary to describe the denaturation process in detail. ESR and absorbance measurements have shown that structural changes of the copper environment occur during the protein denaturation. In particular, the geometry of the copper-ligand atoms changes from being tetrahedral to square planar and the disruption of the active site precedes the global protein denaturation. The thermodynamic enthalpic change, the half-width transition temperature, and the value of ΔCp, were used to calculate the thermodynamic stability, ΔG, of the reversible process over the entire temperature range of denaturation. The low thermal stability found for plastocyanin, is discussed in connection with structural factors stabilizing the native state of a protein. Received: 17 July 1997 / Revised version: 22 November 1997 / Accepted: 15 January 1998     

Activation of mechanisms of photoprotection by desiccation and by light: poikilohydric photoautotrophs

Mechanisms of protection against photo-oxidation in selected desiccation-tolerant lichens and mosses have been investigated by measuring loss of light absorption during desiccation and chlorophyll fluorescence as indicators of photoprotection. Apparent absorption (1-T) spectra measured in the reflectance mode revealed stronger absorption of photosynthetic pigments in hydrated than in desiccated organisms, but differences were pronounced only in a cyanolichen, less so in some chlorolichens, and even less in mosses. Since the amplitude of chlorophyll fluorescence is a product of (1-T) light absorption by chlorophyll and quantum yield of fluorescence, and since fluorescence is inversely related to thermal energy dissipation, when chemical fluorescence quenching is negligible, fluorescence measurements were used to measure changes in energy dissipation. Preincubation of the hydrated organisms and desiccation in darkness excluded the contribution of mechanisms of energy dissipation to photoprotection which are dependent on the presence of zeaxanthin or on the light-dependent formation of a quencher of fluorescence within the reaction centre of photosystem II. Fast drying in darkness or in very low light was less effective in decreasing chlorophyll fluorescence than slow drying. Heating the desiccated organisms increased fluorescence by inactivating the mechanism responsible for fluorescence quenching. Glutaraldehyde inhibited fluorescence quenching during desiccation. Prolonged exposure of a desiccated moss or a desiccated lichen to very strong light caused more photo-induced damage after fast drying than after slow drying. The photo-oxidative nature of damage was emphasized by the observation that irreversible loss of fluorescence was larger in air than in a nitrogen atmosphere. It is concluded from these observations that desiccation-induced conformational changes of a chlorophyll protein complex result in the fast radiationless dissipation of absorbed light energy. This mechanism of photoprotection is more effective in preventing photo-oxidative damage than other mechanisms of energy dissipation which require light for activation such as zeaxanthin-dependent energy dissipation or quencher formation within the reaction centre of photosystem II.     

A flat inclined modular photobioreactor for outdoor mass cultivation of photoautotrophs

A flat inclined modular photobioreactor (FIMP) for mass cultivation of photoautotrophic microorganisms is described. It consists of flat glass reactors connected in cascade facing the sun with the proper tilt angles to assure maximal exposure to direct beam radiation. The optimal cell density in reference to the length of the reactor light path was evaluated, and the effect of the tilt angle on utilization of both direct beam as well as diffuse sunlight was quantitatively assessed. The mixing mode and extent were also optimized in reference to productivity of biomass. The FIMP proved very successful in supporting continuous cultures of the tested species of photoautotrophs, addressing the major criteria involved in design optimization of photobioreactors: Made of fully transparent glass, inclined toward the sun and endowed with a high surface-to-volume ratio, it combines an optimal light path with a vigorous agitation system. The maximal exposure to the culture to solar irradiance as well as the substantial control of temperature facilitate, under these conditions, a particularly high, extremely light-limited optimal cell density. The integrated effects of these growth conditions resulted in record volumetric and areal output rates of Monodus subterraneus, Anabana siamensis, and Spirulina platensis. (c) 1996 John Wiley & Sons, Inc.     

The relationship of the processes of recovery from thermal and radiation damage in yeast cells

Synergistic enhancement coefficient (SEC) achieves the maximum value within 2-3 hours of holding overheated (50 degrees C) and gamma-irradiated diploid cells of Sacch. cerevisiae in water at 28 degrees C; further holding leads to SEC dropping almost to the initial value. The maximum in the curve involved disappears if gamma-irradiated (overheated) cells were held in water at 28 degrees C for 2-3 hours before hyperthermic treatment (gamma-irradiation). There is no maximum in the curve discussed for haploid yeast cells, which are incapable of "rapid" post-irradiation recovering. The experimental data are interpreted from the point of view of reversible inhibition of yeast post-irradiation recovery by hyperthermia.     

Optimizing thermal and radiation effects for bacterial inactivation

The temperatures required for dry-heat spacecraft sterilization have been known to degrade heat-sensitive components. Thermoradiation, the simultaneous application of dry heat and gamma radiation, can provide the same degree of microbial inactivation as dry heat alone while substantially reducing component degradation. This is made possible by the synergistic effects produced when relatively low levels of these agents (e.g., 90 to 350 krads and 60° to 105°C) are applied simultaneously, thus permitting the use of lower temperatures and a reduced duration of heat exposure. The effects of temperature, radiation dose rate, and relative humidity on microbial inactivation during thermoradiation exposure have been established.This experimentation was supported by NASA Contract No. W-12853.     

Thermodynamics of Energy Production from Biomass

With high quality petroleum running out in the next 50 years, the world governments and petrochemical industry alike are looking at biomass as a substitute refinery feedstock for liquid fuels and other bulk chemicals. New large plantations are being established in many countries, mostly in the tropics, but also in China, North America, Northern Europe, and in Russia. These industrial plantations will impact the global carbon, nitrogen, phosphorus, and water cycles in complex ways. The purpose of this paper is to use thermodynamics to quantify a few of the many global problems created by industrial forestry and agriculture. It is assumed that a typical tree biomass-for-energy plantation is combined with an efficient local pelleting facility to produce wood pellets for overseas export. The highest biomass-to-energy conversion efficiency is afforded by an efficient electrical power plant, followed by a combination of the FISCHER-TROPSCH diesel fuel burned in a 35%-efficient car, plus electricity. Wood pellet conversion to ethanol fuel is always the worst option. It is then shown that neither a prolific acacia stand in Indonesia nor an adjacent eucalypt stand is “sustainable.” The acacia stand can be made “sustainable” in a limited sense if the cumulative free energy consumption in wood drying and chipping is cut by a factor of two by increased reliance on sun-drying of raw wood. The average industrial sugarcane-for-ethanol plantation in Brazil could be “sustainable” if the cane ethanol powered a 60%-efficient fuel cell that, we show, does not exist. With some differences (ethanol distillation vs. pellet production), this sugarcane plantation performs very similarly to the acacia plantation, and is unsustainable in conjunction with efficient internal combustion engines.     

Thermodynamics of unfolding for turkey ovomucoid third domain: thermal and chemical denaturation.

We have used thermal and chemical denaturation to characterize the thermodynamics of unfolding for turkey ovomucoid third domain (OMTKY3). Thermal denaturation was monitored spectroscopically at a number of wave-lengths and data were subjected to van't Hoff analysis; at pH 2.0, the midpoint of denaturation (Tm) occurs at 58.6 +/- 0.4 degrees C and the enthalpy of unfolding at this temperature (delta Hm) is 40.8 +/- 0.3 kcal/mol. When Tm was perturbed by varying pH and denaturant concentration, the resulting plots of delta Hm versus Tm yield a mean value of 590 +/- 120 cal/(mol.K) for the change in heat capacity upon unfolding (delta Cp). A global fit of the same data to an equation that includes the temperature dependence for the enthalpy of unfolding yielded a value of 640 +/- 110 cal/(mol.K). We also performed a variation of the linear extrapolation method described by Pace and Laurents, which is an independent method for determining delta Cp (Pace, C.N. & Laurents, D., 1989, Biochemistry 28, 2520-2525). First, OMTKY3 was thermally denatured in the presence of a variety of denaturant concentrations. Linear extrapolations were then made from isothermal slices through the transition region of the denaturation curves. When extrapolated free energies of unfolding (delta Gu) were plotted versus temperature, the resulting curve appeared linear; therefore, delta Cp could not be determined. However, the data for delta Gu versus denaturant concentration are linear over an extraordinarily wide range of concentrations. Moreover, extrapolated values of delta Gu in urea are identical to values measured directly.     

Biogeographical patterns of abundant and rare bacterial biospheres in paddy soils across East Asia

Soil bacterial communities play fundamental roles in ecosystem functioning and often display a skewed distribution of abundant and rare taxa. So far, relatively little is known about the biogeographical patterns and mechanisms structuring the assembly of abundant and rare biospheres of soil bacterial communities. Here, we studied the geographical distribution of different bacterial sub-communities by examining the relative influence of environmental selection and dispersal limitation on taxa distributions in paddy soils across East Asia. Our results indicated that the geographical patterns of four different bacterial sub-communities consistently displayed significant distance–decay relationships (DDRs). In addition, we found niche breadth and dispersal rates to significantly explain differences in community assembly of abundant and rare taxa, directly affecting the strength of DDRs. While conditionally rare and abundant taxa displayed the strongest DDR due to higher environmental filtering and dispersal limitation, moderate taxa sub-communities had the weakest DDR due to greater environmental tolerance and dispersal rate. Random forest models indicated that soil pH (9.13%–49.78%) and average annual air temperature (16.59%–46.49%) were the most important predictors of the variation in the bacterial community. This study advances our understanding of the intrinsic links between fundamental ecological processes and microbial biogeographical patterns in paddy soils.     

Comparative analysis of strategies to prepare electron sinks in aquatic photoautotrophs

Photosynthesis Research - While subject to illumination, photosystem I (PSI) has the potential to produce reactive oxygen species (ROS) that can cause photo-oxidative damage in oxygenic...     

Thermodynamics of partitioning and efflux of phenothiazines from liposomes

Summary The partitioning of nine phenothiazines between dimyristoylphosphatidylcholine (DMPC) liposomes and 0.9% wt/vol saline at pH 6 has been studied both below and above the phase transition temperature (T_c) of the phospholipid. Higher partitioning was observed aboveT_c. Both the entropy and enthalpy of partitioning were positive below and aboveT_c, and a linear relationship between the entropy and enthalpy has been derived. In general, the partitioning and transport of alkylaminophenothiazines in DMPC liposomes over the temperature range of 5 to 40°C is entropically controlled. The entropies and enthalpies of partitioning of various groups in the phenothiazine structure have been calculated.No relationship was found between particle size of the DMPC liposomes and the equilibrium partition coefficient at 25°C. However, the particle size of liposomes did increase with increasing acyl chain length of the phospholipid.Using differential scanning calorimetry, the enthalpy and entropy of transition of the DMPC liposomes in the absence and presence of phenothiazines has been calculated. The temperature dependence of the first-order rate constant of trimeprazine tartrate transport in DMPC liposomes was investigated and was found to be maximum at theT_c of the phospholipid.     

The effects of solar radiation on thermal comfort

The aim of this study was to investigate the relationship between simulated solar radiation and thermal comfort. Three studies investigated the effects of (1) the intensity of direct simulated solar radiation, (2) spectral content of simulated solar radiation and (3) glazing type on human thermal sensation responses. Eight male subjects were exposed in each of the three studies. In Study 1, subjects were exposed to four levels of simulated solar radiation: 0, 200, 400 and 600 Wm⁻². In Study 2, subjects were exposed to simulated solar radiation with four different spectral contents, each with a total intensity of 400 Wm⁻² on the subject. In Study 3, subjects were exposed through glass to radiation caused by 1,000 Wm⁻² of simulated solar radiation on the exterior surface of four different glazing types. The environment was otherwise thermally neutral where there was no direct radiation, predicted mean vote (PMV)=0±0.5, [International Standards Organisation (ISO) standard 7730]. Ratings of thermal sensation, comfort, stickiness and preference and measures of mean skin temperature (t_sk) were taken. Increase in the total intensity of simulated solar radiation rather than the specific wavelength of the radiation is the critical factor affecting thermal comfort. Thermal sensation votes showed that there was a sensation scale increase of 1 scale unit for each increase of direct radiation of around 200 Wm⁻². The specific spectral content of the radiation has no direct effect on thermal sensation. The results contribute to models for determining the effects of solar radiation on thermal comfort in vehicles, buildings and outdoors.     

Synchrotron radiation small angle scattering studies of thermal stability of xylanase XYNII from Trichoderma longibrachiatum

Xylanase XYNII from Trichoderma longibrachiatum is a small protein of the molecular weight 21 kDa, belonging to the family 11 of glycosyl hydrolases, which catalyses hydrolysis of xylan. This article reports thermal stability study of xylanase XYN II conformation in the temperature range 15-65 degrees C by the small angle synchrotron radiation scattering. The study has been performed at different pH conditions: at pH 4.0 (below the physiological optimum of the enzyme activity) at pH 5.8 close to the optimum for enzymatic activity and at pH 8.0. The radius of gyration and the pair distance distribution function p(r) have been analyzed to characterize the changes of the enzyme conformation on heating. In the environment of the pH close to that of the optimum for the enzymatic activity, xylanase shows the greatest thermal stability and undergoes denaturation only above 55 degrees C. In the acidic and basic environments, the enzyme stability is much lower and denaturation begins at 45 degrees C. On the basis of the SAXS data, the shape of the xylanase molecule in solution in different temperatures has been reconstructed using ab initio method and program DAMMIN. The shape of the xylanase molecule at room temperature is similar to the right hand, which is typically observed for xylanase crystal structure. In higher temperatures (close to the enzyme activity optimum), the conformation of the right hand is loosened and half opened.     

Thermodynamics of thermal and athermal denaturation of gamma-crystallins: changes in conformational stability upon glutathione reaction

The conformational stabilities of bovine lens gamma-crystallin fractions II, IIIA, IIIB, and IVA and those modified with glutathione were compared by studying the thermal and guanidine hydrochloride (Gdn-HCl) denaturation behavior. The conformational state was monitored by both far-UV CD and fluorescence measurements. All the gamma-crystallins studied showed a sigmoidal order-disorder transition with varied melting temperatures. The thermal denaturation of these proteins is reversible up to a temperature 3 or 4 degrees C above T 1/2; above this temperature, irreversible aggregation occurs. The validity of a two-state approximation of both thermal and Gdn-HCl denaturation was tested for all four crystallins, and the presence of one or more intermediates was evident in the unfolding of IVA. delta GDH2O values of these crystallins range from 4 to 9 kcal/mol. Upon glutathione treatment IVA showed the maximum decrease in T 1/2 by approximately 9 degrees C and in delta GDH2O value by 29%; the smallest decrease in T 1/2 was for IIIA by 2 degrees C and in delta GDH2O by 15%. We have demonstrated that the glutathione reaction can dramatically reduce the conformational stability of gamma-crystallins and, thus, that the thermodynamic quantities of the unreacted crystallins can be used to evaluate the stability of these proteins when modified during cataract formation.